The Burj Khalifa's observation deck is worth booking ahead, especially for a sunset slot. The Dubai Fountain show outside the mall is free and runs every 15-30 minutes in the evening. A visit to the Gold Souk and Spice Souk in Deira, followed by an abra boat ride across the creek, gives you a genuinely different side of the city from the glass towers downtown.
A desert safari β dune driving, a camel ride, and dinner under the stars β is one of the more popular add-ons for visitors who want to see beyond the city itself.